Field of the Invention
The present invention relates to a galling and corrosion resistant inner diameter surface in aluminum caster roll shell steels. Particularly, the invention relates to the treatment and final product of the inner diameter surface of a caster roll shell through a nitriding process.
Discussion of the Related Art
Hollow, cylindrical steel roll shells that are used in the strip casting of aluminum (Al) and other lower melting metals are shrink fitted on a cylindrical solid steel core. The core outer diameter surface can be cylindrically grooved to provide a water channel path for cooling of the roll shell. The core hardness has typically been higher than that of the roll shells. The core is typically manufactured by weld overlaying a typical AISI 4130 core with 420 SS (up to 50 to 56 HRc) which can then be machined to generate the cylindrical water channels. An exemplary roll shell-core configuration includes a roll set with two roll shells, each around a core, that act as a die that solidifies the lower temperature metal such as Al. Molten metal, e.g. Al, enters the revolving roll set on one side and emerges as a solid wide thin strip on the exit side.
Because the steel roll shell is shrink fit onto the steel core, the roll shell tends to move slowly on the core during the casting operation. It is important, therefore, that the internal diameter surface of the roll shell has sufficient galling resistance to minimize any metal debris produced by the surface-to-surface grinding movement. In current applications metal debris is generated over time and accumulates in the water channels that may be provided onto the steel core. The debris can restrict and sometimes block the cooling water flow. This can directly affect the uniform heat extraction by the roll shell-core rolls and result in wide variation in Al strip thickness. As a consequence, the further cold rolling reduction of the strip to uniform sheet becomes very difficult if not impossible to accomplish.
Many attempts to reduce galling and debris generation in the core coolant channels have been explored. The most obvious solution has been to increase the interference shrink fit between the roll shell and core so that the slippage which causes the galling and debris generation is greatly reduced. However, this causes earlier generation of outer diameter surface heat checking or cracking as a result of the exceedingly higher circumferential tension stresses. Thus, interference shrink fit can often lead to a reduced service life of the roll set and can also result in early catastrophic failure.
One example of mechanically enhancing the interference shrink fit is disclosed in U.S. Patent Application Publication No. 2005/039875, which is incorporated herein by reference. In this application, a steel core and a copper roll shell are provided with protuberances and indentations oriented at least partially in the direction of the casting roll axis and extending radially for at least 2 μm. Although this process may lead to reduced galling, the rough mechanical bonded surfaces of the core and roll shell can potentially reduce the rate of heat transfer and thus not necessarily improve the overall casting process.
Increasing the hardness of the overall steel roll shell by going to higher carbon steel alloys and alloying to produce high temperature yield strength has been used to develop a greater resistance to heat checking. An example of such approach is provided in U.S. Pat. No. 4,409,027, which is incorporated herein by reference. The higher hardness associated with these higher strength roll shells (46 to 48 HRc) is somewhat resistant to galling damage at the roll shell-core interface. However, the results are not ideal and the increased brittleness associated with higher strength alloys results in less shrink fit, which has been known to increase movement at the roll shell-core interface and thus potentially increase the generation of debris. Another disadvantage of using increased hardness roll shell is that it requires more care in handling the roll shells during installation and intermediate surface conditioning to avoid cracking and breakage.
Selected alloy content increases in lower carbon steel roll shells, as exemplified in U.S. Pat. No. 5,599,497, which is incorporated herein by reference, has been very effective in reducing the onset of heat checking through high temperature strengthening by alloy carbide and nitride precipitation hardening that also allows for greater shrink fit to reduce roll shell-core movement. However, this approach does not address the problem of galling on the roll shell inner diameter surface because of the slightly lower hardness 44 to 46 HRc. Also, the higher alloy content can reduce the thermal conductivity in the roll shell and its ability to withdraw heat from the incoming molten Al.
Recently, in U.S. Pat. No. 8,303,892, which is incorporated herein by reference, it has been proposed that the alloy content be decreased even further to promote faster strip production rates by virtue of maximizing heat conductivity in the roll shell. However, again this has come at the cost of slightly lower roll shell hardness levels (42 to 44 HRc) and thus the potential for greater debris generation at the roll shell-core interface.
A commonly used procedure to address the above problems is to add a hard metal coating such as Cr (up to 0.004 in. thick and 58 to 62 Rc) or other hard elements to the inner diameter surface of the roll shell to increase abrasion resistance at the roll shell-core interface. This process, however, is limited to low temperature applications so as not to interfere with the mechanical properties of the roll shell. This limits the process to electrolytic plating as for example described in U.S. Pat. No. 5,265,332, which is incorporated herein by reference, that describes the procedure and its benefits. Through this coating process, abrasion galling resistance can initially be improved at the roll shell-core interface because the Cr coated steel roll shell can slide more easily over the core during casting. However, it has been shown that the brittle Cr plating often can contain numerous cracks or fissures that can break loose and cause debris buildup in the water channel. Also, the Cr plating can act as a barrier to heat extraction from the Al strip solidification thus slowing down the casting speed.
Another deleterious problem frequently experienced with Cr plating is the onset of galvanic corrosion of the roll shell under these Cr plates. This is caused by the coolant water seeping through the cracks in the Cr plate to the roll shell steel. Because of this effect, cracks can form in the roll shell inner diameter surface. The cracks often can initiate in the roll shells at the crevices where the core edges, at the water channels, meet the roll shell inner diameter surface. The corrosion rate at these crevices that become exposed to the coolant water flowing through the core channels is accelerated by the 100% Cr plated surfaces that drive the corrosion, by the area effect, to the less noble roll shell steel (usually 2% to 4% Cr in typical roll shells). When a roll shell crack is formed it can easily spread by the cyclic stresses during casting from the inner diameter surface roll shell surface outward to the roll shell outer diameter surface. This can lead to the roll shell springing water leaks or to catastrophic failure during the casting operation.
Attempts have been made to minimize the cracking by adding inhibitors to control the pH of the coolant water but have not been very effective because the inhibitors often increase the conductivity of the solution to 300-400 MMHOS/cm or higher, and thus promote galvanic corrosion especially at the core edges-roll shell inner diameter surface crevices.
Attempts have also been made to reduce the conductivity of the coolant water by using de-ionized water. However, this is a very expensive process that is nearly impossible to attain in commercial water coolant systems.